1. Field of the Invention
This invention relates to microbial reactors and to microbial reactor systems in general, and more specifically, to a mixer/contactor for a slurry, suspension or settled bed reactor system. The mixer/contactor is advantageous for processes wherein contact time may be relatively long, and/or wherein minimum energy input for suspending and mixing solid particles in the slurry, suspension or settled bed is desired, for example, in the biological remediation of liquid waste streams or contaminated sludges or soils.
2. Related Artxe2x80x94Soil Remediation
Slurry or suspension or settled bed reactors are commonly used for processing ores, soils, and wood chips. Also, they are commonly used to effect the biological, enzymatic or chemical conversion of soluble and insoluble reactants. A slurry or suspension is a mixture of a liquid and solid particles, wherein at least a portion of the solid particles are suspended in the liquid. In a slurry or suspension reactor, a portion of the solid particles may be settled in the bottom of the reactor to form a bed.
There is a large need for conversion of contaminants, especially chemical contaminants, found in liquid waste streams, sludges and soils. These chemicals may be organic or inorganic, and hazardous or toxic. Many millions of cubic feet of soils contaminated with these chemicals exist throughout the world and are in need of remediation.
Biodegradation of many of these contaminating chemicals has been conducted. xe2x80x9cBiodegradationxe2x80x9d means breaking down these chemicals to less hazardous or less toxic reaction products via biological pathways using microorganisms. The microorganisms, or xe2x80x9cmicrobesxe2x80x9d, may operate aerobically, microaerophylically or anaerobically, or in any combination thereof. Also, the microorganisms may operate via oxidative pathways or reductive pathways. Microorganisms include bacteria, protozoa, fungi and algae. Biodegradation of soils contaminated with chemicals is one way to remediate the soil.
Often, the remediating microorganisms operate on the soil contaminating chemicals in a slurry environment in a reactor vessel, wherein the soil is mixed with water to at least partially suspend the soil particles for intimate contact with the microorganisms. To further increase suspension, mixing and contacting, a gas, such as air in aerobic applications, for example, may be added to the reactor vessel. When the reactor vessel contains microorganisms and a slurry, it is referred to as a bioslurry reactor.
Presently, at least four bioslurry reactor systems are being commercialized for soil remediation. The first system, developed by MOTEC, Inc. of Mt. Juliet, Tenn., involves technology adapted for treatment of pesticides, PCB""s, dioxin and halogenated and nonhalogenated organic compounds. While demonstrated to be effective for treating sludge, liquids and soils having high organic concentrations, the MOTEC process has been reported to be less suitable for use with inorganic-laden wastes.
The MOTEC technology, which is a sequential process, is also referred to as liquid solid contact digestion (LSCD). The system involves two to three tank digestor which are aerated using air spargers and are agitated using turbine mixers. Alternatively, this technology may be adapted, by use of high shear propeller mixers, to enhance aerobic biological degradation in lagoons.
The second technology, developed by Detox Industries, Inc. of Sugarland, Tex., is intended for use in treating chlordane, myrex, oil, phenolics, polycyclic aromatic hydrocarbons, creosote, pentachlorophenol (PCP) and polychlorinated biphenyls (PCB""s). The Detox system includes an open-topped reaction tank or on-site created lagoon that utilizes a synthetic liner. The tank is adapted to retain a slurry and is fitted with air distributors.
Another bioslurry reactor, consisting of several agitated and aerated vessels, has been used in a pesticide spill application by ECOVA of Redmund, Wash.
The MOTEC, Detox, and ECOVA systems described above are operated in batch mode. After the placement of contaminated soil and water into the reactor vessel, the vessel is aerated until a desired residual contaminant level is reached, and then the supernatant water is usually recycled and the slurry is discharged. Due to the ongoing aeration in these systems, many volatile organic substances are not biodegraded but rather are air-stripped. Some systems treat these air-stripped volatiles in a carbon adsorption filter whereas other systems simply discharge them to the atmosphere.
A fourth system, known as the EIMCO Biolift(copyright) system, utilizes a bioreactor that is a tank having a bottom, upstanding walls fixedly mounted thereon and a sealed top or cover, and which is adapted to receive and contain a slurry. The tank is fitted with a mechanical mixing means that operates to effectuate agitation and suspension of the solid particles within the slurry housed within the tank. An air supply operates to provide oxygen, which is a necessary component of the biooxidation reaction taking place within the bioreactor. The air supply also is configured to provide suspension of the particulate within the slurry liquid housed within the tank. In addition, an air lift is provided for recirculating particulate which may have settled out of the slurry. The Biolift(copyright) system may be operated in continuous mode by using a screening device and exit conduit located near the top inside the tank to draw off treated water and excess particulate matter.
Considerable literature is available describing slurry reactors for municipal and farm sewage digestion, but the total solids for these applications are usually below 10 wt %. The density of sewage sludges is much closer to the density of water than is the density of soil, and therefore the mixing method and design of these sewage sludge stirred reactors can be significantly different than that of soil-slurry reactors. Many sewage digester designs are unstirred, and the predominant mixing mechanism is the CO2 and CH4 gas generated in the reactor. The mixing occurs as these gas bubbles rise through the slurry. Propeller type mixers are sometimes added for more thorough mixing and to try to maintain the solids in suspension. The current design of most soil-slurry reactors is to finely pulverize the material and try to keep it in suspension with significant power input to shaft stirrers, aerators, recirculation pumps or a combination of these methods. The alternative approach is to not mix at all or to mix only occasionally. With the extended residence time required for most biodegradation, there is probably no need for a high shear or complete suspension agitation, especially for an anaerobic design.
In aerobic soil-slurry reactors it is difficult to maintain high oxygen concentrations due to the tendency for gas bubbles to coalesce. Also, since the reactors are usually low in profile, there is a very short liquid-gas contact time and a small surface area to volume ratio of the bubbles. Mechanical agitation is usually required to disperse gas bubbles and give smaller gas bubbles, but as the solids concentration increases the agitation effect decreases.
Common to all hazardous waste treatment systems utilizing microorganism activity is the requirement of providing an adequate supply of nutrients to the microorganisms. This provision allows biomass growth and facilitates the occurrence of biochemical reactions. Various approaches have been used to optimize bioactivity level in reactor vessels. In those systems wherein a multiplicity of connected reactor vessels have been suggested, e.g. cascade systems, a common problem is the retention and maintenance of biomass in a given reactor as effluent from the reactor is directed to the next reactor.
The clean-up of hazardous waste sites requires innovative approaches that are cost effective. Biological systems can play an important role in soil bioremediation, as they have in the field of wastewater treatment. In order to be cost effective in contaminated soil treatment, however, bioreactor vessels and processes are needed that can handle high solids concentrations and large throughput volumes with a minimum of input and/or operating energy.
3. Related Artxe2x80x94Wastewater Remediation
Dissolved organic matter and suspended solids are often removed from wastewaters by a combination of biodegradation and filtration. Conventional designs for microbial water treatment processes are based on suspended microorganisms to degrade organic matter in wastewaters. These activated sludge processes are in wide usage, but they are not efficient in removing waste materials, and they require large facilities. Attached-growth systems, with bioreactors packed with inert media on which microorganisms can attach and grow, are much more efficient than suspended growth systems.
A variety of arrangements have been used to clean wastewaters using biological treatment and filtration. One arrangement has been bioreactors in line with sedimentation tanks and filters. The reactor can be as simple as a tank where air is injected and aerobic bacteria are grown on inert carriers such as plastics and sand. U.S. Pat. No. 5,007,620 discloses a bioreactor equipped with a sweeping means adapted for sweeping and scouring the bottom of the bioreactor. Stationary diffusers are used to aerate the bioreactor. By these means, an aerated slurry can be maintained by means of mechanical agitation and aeration.
Alternatively, a bioreactor can be operated anaerobically in processes such as anaerobic digestion and denitrification. U.S. Pat. No. 3,970,555 discloses a method for backwashing a filter by injecting a fluid, such as air or water, to dislodge gas bubbles trapped in a filter bed. Such backwashing also removes solids clogging the filter bed.
Bioreactors operated with inert media for supporting microbial growth, are termed xe2x80x9cbiofiltersxe2x80x9d, and have been operated with a stationary filter medium or a movable filter medium. The filter medium typically consists of plastic materials or inorganic materials such as sand. Stationary filter media must be periodically cleaned by a reversed-flow washing, termed xe2x80x9cbackwashingxe2x80x9d. Backwashing, especially when done with injected water and air, can effectively clean the medium. However, the backwash fluids must be collected and treated. Also, the biofilter must be periodically removed from service in order to do the backwashing.
Fluidized bioreactors have been developed for continuous operation without backwashing (U.S. Pat. No. 5,007,620). However, such bioreactors require continuous agitation with high energy consumption. In addition, fluidized bioreactors cannot clean wastewater by filtration.
The instant invention is a slurry mixer/contactor for a slurry, suspension or settled bed reactor system. The mixer/contactor is particularly well-suited for the biological remediation of liquid waste streams or contaminated sludges or soils. The reactor system having the mixer/contactor of this invention may operate with microorganisms living in the reactor aerobically or anaerobically. Also, the reactor system may operate with microorganisms in the reactor using oxidative or reductive pathways to biodegrade contaminants. To further increase activity in the bioreactor, additional ingredients, including solids, liquids or gases, may be added to the slurry, suspension or settled bed in the reactor.
The reactor having the mixer/contactor in one embodiment is an upright generally cylindrical vessel with a flat bottom and a covered top. In the inside of the vessel, preferably along the cylinder centerline, is a vertical conduit, also called a supply conduit, extending from near the top to near the bottom. At the bottom of the vertical conduit is the inlet to at least one generally horizontal, stirrer blade in fluid connection with the vertical conduit. The stirrer blade has outlet openings in it or on it so fluid may pass there-through. The stirrer blade may rotate around the vertical conduit if the conduit is fixed, or the conduit, with the stirrer blade fixed to it, may rotate around in the vessel. In any event, the rotation of the stirrer blade is caused or made easier by the hydraulic forces of fluid flowing out from the stirrer blade. This rotation may be caused or made easier by a fluidization effect, by a jet propulsion effect, or both. This way, rotation of the stirrer blade may be created or eased near the bottom of the vessel, enhancing mixing of the microorganisms with the sludge or soil in the slurry, suspension or settled bed in the vessel, without unnecessarily damaging the microorganisms and without having to fluidize the complete vessel contents. Also, this way liquid and/or slurry from near the top of the reactor may be re-distributed into the sediment near the bottom of the reactor for fresh re-contact and further desorption and reaction of contaminants from the sediment to permit further biodegradation.
In a preferred embodiment, the bioreactor vessel has a water recycle outlet port and a gas recycle outlet port. At the water recycle outlet port, which is below the fill line for the reactor, is a water recycle outlet conduit leading to the inlet of a water recycle pump. The pump delivers recycled water back to the bioreactor vessel through a water inlet conduit connected to the vessel at a water recycle inlet port. Preferably, the water inlet port is connected to the vertical conduit in the center of the vessel, and the flow of recycled water helps to provide the hydraulic forces for fluidizing the sediment in the immediate vicinity of the blade and/or the jet propulsion that results in rotation of the stirrer blade.
At the gas recycle outlet port of the bioreactor vessel is a gas recycle outlet conduit leading to the inlet of a gas recycle pump or compressor. The compressor may deliver recycled gas and/or fresh gas to the bioreactor vessel through a gas inlet conduit connected to the vessel at a gas inlet port, or connected to the water inlet conduit. This way, water and gas from the bioreactor vessel may be recycled and provided to the vertical conduit to help create rotation of the stirrer blade, and better mixing and contacting of the slurry or suspension and the microorganisms in the bioreactor vessel.
Preferably, the mixer/contactor is one, generally horizontal blade in fluid connection with the vertical conduit. The blade has outlet openings at or near its leading edge so that fluid passing through the vertical conduit and to the blade may pass from the blade through the openings. This way, a fluidization zone is created in the slurry, suspension or settled bed of solid particles, in the region of the openings at or near the leading edge of the blade. By xe2x80x9cleading edgexe2x80x9d of the blade is meant the front edge or side of the blade relative to the direction of rotation of the blade. By xe2x80x9ctrailing edgexe2x80x9d is meant the back edge or side of the blade relative to rotation. More than one blade may be used, and blades at various depths in the reactor may also be used. When a plurality of blades is used, they may rotate independently of, or together as a unit with, the other blade(s).
The fluidization zone created by the flow of fluid from the outlet openings in the blade has less density than the rest of the slurry, suspension or settled bed throughout the reactor. Therefore, any generally horizontal force on the blade will tend to cause it to rotate into the fluidization zone. This horizontal force on the blade may be created by a propulsion jet at the trailing edge of the blade. Or, this horizontal force may be created by the horizontal component of the weight vector from settling sediment on a rear, downwardly sloping portion of the blade as the sediment descends from a fluidized state to a settled state at the trailing edge of the blade. Or, this horizontal force may be created by the input from an external power source, like an electric motor, for example, connected to and rotating the vessel""s vertical conduit. Therefore, the blade rotates, or its rotation in the reactor vessel is made easier, due to the hydraulic forces of fluid flowing out from the stirrer blade.
This invention also relates to the use of a single bioreactor to achieve both biological degradation and the filtration of suspended solids. The mixing/contacting blade intermittently suspends the filtration medium as the blade rotates through the medium to efficiently backwash and clean it.
One objective of this invention is to provide an efficient biofilter equipped with a filter bed through which movable blades can be propelled by the injection of fluids, such as air and water. Thereby, the filter bed can be intermittently suspended and efficiently cleaned of trapped solids and gases. Another objective is to provide a biofilter capable of both microbial degradation of wastewater contaminants and contaminant removal by filtration.